Multilayered Sulfur Composite Cathodes for Lithium Sulfur Batteries

ABSTRACT

A multilayered cathode for a lithium sulfur battery comprising at least one current collector working electrode having a surface comprising a carbon containing layer, two or more sulfur containing layers wherein at least one of the sulfur layers is located in juxtaposition to and in communication with the carbon containing layer, and at least one outermost layer comprising a positively charged polymer for forming interconnected layers of the sulfur containing layer, the carbon containing layer, and the polymer. Preferably, the cathode has layers that are alternatively arranged of two or more different sulfur containing layers. A lithium sulfur battery is provided and a method of making a multilayered cathode for a lithium sulfur battery is disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This utility patent application claims the benefit of priority toco-pending U.S. Provisional Patent Application Ser. No. 62/090,654,filed Dec. 11, 2014. The entire contents of U.S. Provisional patentApplication Ser. No. 62/090,654 are incorporated by reference into thisutility patent application as if fully rewritten herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a multilayered cathode for lithium sulfurbatteries. Specifically, this invention provides a multilayered sulfurcomposite/compound cathode for a lithium sulfur battery, methods toprepare the multilayered sulfur composite/compound cathode, and alithium sulfur battery having the cathode.

2. Brief Description of the Background Art

Lithium sulfur batteries (LSBs) suffer from problems like severecapacity fading and low power density due to high solubility ofintermediate polysulfides and low conductivity of sulfur. Various sulfurcomposites including sulfur carbons and sulfur polymers have beenreported, aiming to achieve fast reaction kinetics and effectivetrapping of soluble polysulfides. Unfortunately, these batteries havetheir own limitations, none of them could resolve the above twochallenges. The best way to improve the performances of rechargeableLSBs is to reduce the dissolution of intermediate polysulfides whileprovide an excellent ionic- and electric-conductive network.

SUMMARY OF THE INVENTION

Considering the advantages of multilayered cathodes including (1) highenergy densities; (2) high power densities and (3) longer cycliclifetimes, the present invention provides multilayered sulfur compositecathodes containing at least two kinds of carbon materials-(functionalized, but not limit to) or polymer materials- (with surfacefunctional groups, but not limit to) based sulfur composites or sulfurcompounds. The novelty of this invention lies mainly in the developmentand application of a new kind of multilayered cathode for lithium sulfurbatteries. In this invention, new sulfur based composite multilayeredcathodes are prepared via highly efficient (1) layer-by-layer (LbL)method, (2) step-by-step electrophoretic deposition (EPD) method, (3)spin-assisted assembly technique and (4) alternately misting method. Theadvantages of the methods of the present invention are (1) costeffectiveness, (2) simplicity, and (3) eco-friendly manufacturing. Thisinvention provides multilayered cathodes containing alternativelyarranged two or more different sulfur composite/compound layers withporous structures produced by binder free highly cross-linked selectedmaterials.

An embodiment of this invention provides a multilayered cathode for alithium sulfur battery comprising at least one current collector workingelectrode having a surface comprising a carbon containing layer, two ormore sulfur containing layers wherein at least one of said sulfur layersis located in juxtaposition to and in communication with the carboncontaining layer, and at least one outermost layer comprising apositively charged polymer for forming interconnected layers of thesulfur containing layer, the carbon containing layer, and the polymer,wherein the outermost layer is in juxtaposition to and in communicationwith at least one of the sulfur layers. The multilayered cathode of thisinvention preferably includes wherein the carbon containing layercomprises a carbon nanotube —COO⁻ moiety. The multilayered cathode ofthis invention comprising alternatively arranged layers of the sulfurcontaining layers wherein the sulfur containing layers comprise one ormore sulfur containing compounds and one or more sulfur-carbon-polymercomposites. Preferably, the multilayered cathode of this inventionincludes wherein the layers have porous cross-linked structures.

Another embodiment of this invention provides the multilayered cathode,as described herein, wherein at least one of the sulfur containinglayers is a sulfur-carbon nanotube polystyrene sulfonate polymer.

Another embodiment of this invention provides the multilayered cathode,as described herein, wherein the outermost layer of the positivelycharged polymer is a sulfur polyaniline polymer.

In yet another embodiment of this invention, a lithium sulfur battery isdisclosed having at least one multilayered cathode and at least oneanode, wherein the multilayered cathode comprises at least one currentcollector working electrode having a surface comprising a carboncontaining layer; two or more sulfur containing layers wherein at leastone of the sulfur layers is located in juxtaposition to and incommunication with the carbon containing layer; and at least oneoutermost layer comprising a positively charged polymer for forminginterconnected layers of the sulfur containing layer, the carboncontaining layer, and the polymer, wherein the outermost layer is injuxtaposition to and in communication with at least one of the sulfurlayers. Preferably, the lithium sulfur battery, as described herein,includes wherein the carbon containing layer of the multilayered cathodecomprises a carbon nanotube —COO⁻ moiety (i.e. an example of afunctionalized carbon nanotube). Most preferably, the lithium sulfurbattery of this invention, as described herein, includes wherein themultilayered cathode comprises alternatively arranged layers of thesulfur containing layers wherein the sulfur containing layers compriseone or more sulfur containing compounds and one or moresulfur-carbon-polymer composites.

Another embodiment of this invention provides a lithium sulfur battery,as described herein, including wherein the layers of the multilayeredcathode have porous cross-linked structures.

Another embodiment of this invention provides a lithium sulfur battery,as described herein, includes wherein at least one of the sulfurcontaining layers of the multilayered cathode is a sulfur-carbonnanotube polystyrene sulfonate polymer.

Another embodiment of this invention provides a lithium sulfur battery,as described herein, including wherein the outermost layer of thepositively charged polymer of the multilayered cathode is a sulfurpolyaniline polymer.

In yet another embodiment of this invention, a method of making amultilayered cathode for a lithium sulfur battery is provided. Thismethod of making a multilayered cathode for a lithium sulfur batterycomprises employing at least one of the methods selected from the groupconsisting of (1) a layer-by-layer (LbL) method, (2) a step-by-stepelectrophoretic deposition (EPD) method, (3) a spin-assisted assemblytechnique, and (4) an alternately misting method, to produce amultilayered sulfur composite cathode.

Preferably, the method for making a multilayered cathode for a lithiumbattery comprises (a) providing a sulfur carbon nanotubepolystrenesulfonate composition dispersed in water for forming a sulfurcarbon nanotube polystyrene dispersion; (b) providing a sulfurizedpolyaniline composition dispersed in water for forming a sulfurizedpolyaniline dispersion; (c) providing a current collector having asurface comprising a carbon coating; (d) immersing the current collectorhaving the carbon coating into the sulfurized polyaniline dispersion toform a sulfurized polyaniline coated current collector; and (e)immersing the sulfurized polyaniline coated current collector into thesulfur carbon nanotube polystyrene dispersion for forming one layer ofthe sulfurized polyaniline and the sulfur carbon nanotubepolystyrenesulfonate treated current collector; and (f) repeating thesteps (d) and (e) one or more times to form one or more additionallayers of the sulfurized polyaniline and the sulfur carbon nanotubepolystyrenesulfonate upon the treated current collector. This methodincludes wherein the carbon coating comprises one or more of afunctionalized porous carbon, graphite, grapheme, carbon nanoparticles,carbon nanotubes, carbon fibers, and carbon rods. The functionalizedporous carbon is a carbon nanotube functionalized with a COO⁻ group toform a carbon nanotube COO⁻. This method preferably includes wherein thecurrent collector is one or more selected from the group consisting ofan aluminum substrate, a copper substrate, a nickel substrate, and aconductive glass.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 shows a schematic of one embodiment of the method of thisinvention for preparation of a multilayered sulfur composite cathodeemploying the LbL method. The aluminum substrate was treated withnegative charges before the LbL method.

FIG. 2 shows a schematic of one embodiment of the method of thisinvention for preparation of a multilayered sulfur composite cathode ofthis invention employing the LbL method.

FIG. 3 shows scanning electron microscope (SEM) images for carbon-fiberanodes. FIG. 3a shows an SEM image of low magnification (magnificationlevel: ×2000) and FIG. 3b high magnification (magnification level:×10000) for the EPD-grown (electrophoretic deposition grown) carbonnanofiber anodes of this invention.

FIGS. 4a-b show cyclic properties of the multilayered cathodes of thisinvention. FIG. 4a shows cyclic properties of multilayered cathodes atvarious current densities; capacities were calculated based on the wholematerials' weight on cathodes. FIG. 4b shows cyclic properties ofmultilayered cathodes for cycles up to 420 at a current density of 550mAg⁻¹. FIGS. 4c-d show voltage profiles of the Li/S cells of thisinvention. FIG. 4c shows the voltage profiles of discharge/charge cyclesof first and fiftieth cycles of Li/S cells at a current density of 1300mAg⁻¹ and 1950 mAg⁻¹, respectively.

FIGS. 5a and 5b show another embodiment of the multilayered cathode ofthis invention. FIG. 5a is a schematic diagram of multilayered cathodeof this invention fabricated by the LbL process and the functions ofeach component. FIG. 5b is a schematic diagram of the self-controlpoly-shuttle process in the multilayered cathode.

FIGS. 6a-i show characterizations of the multilayered cathodes of thisinvention.

FIGS. 7a-c show rate performance of the multilayered cathodes of thisinvention, and FIGS. 7d-f show SEM images of the multilayered cathodestructure.

FIG. 8a shows a CV scan of the multilayered cathodes of this invention,FIG. 8b shows a voltage profile of the multilayered cathodes of thisinvention, and FIGS. 8c-f show top surface characterization of themultilayered cathodes of this invention at various cycles.

FIG. 9a shows an EUIS analysis of the Li—S cells having the multilayeredcathodes of this invention, and FIG. 9b shows an SEM image of thesurface of the multilayered cathode of this invention after 500 cycles.

FIGS. 10a-c show analysis and spectra, respectively, of the materials ofthe multilayered cathodes of this invention.

FIGS. 11a-c show analysis and spectra, respectively, of the materials ofthe multilayered cathodes of this invention.

FIG. 12 shows cycling performance at various current densities of theslurry coated cathodes of this invention.

FIG. 13 shows CV data of S-CNT and APANI cathodes of this invention.

FIG. 14 shows XRD data of the multilayered cathode of this invention.

FIG. 15 shows continuous CV scans of the multilayered cathodes of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides multilayered sulfur composite cathodescontaining sulfur composites or sulfur compounds. The presentapplication provides a process for making a multilayered cathode forlithium sulfur batteries. In this invention, new sulfur based compositemultilayered cathodes are prepared via highly efficient (1)layer-by-layer (LbL) method, (2) step-by-step electrophoretic deposition(EPD) method, (3) spin-assisted assembly technique and (4) alternatelymisting method. The advantages of the methods of the present inventionare (1) cost effectiveness, (2) simplicity, and (3) eco-friendlymanufacturing. This invention provides multilayered cathodes containingalternatively arranged two or more different sulfur composite/compoundlayers with porous structures produced by binder free highly crosslinkedselected materials.

An embodiment of this invention provides a multilayered cathode for alithium sulfur battery comprising at least one current collector workingelectrode having a surface comprising a carbon containing layer, two ormore sulfur containing layers wherein at least one of said sulfur layersis located in juxtaposition to and in communication with the carboncontaining layer, and at least one outermost layer comprising apositively charged polymer for forming interconnected layers of thesulfur containing layer, the carbon containing layer, and the polymer,wherein the outermost layer is in juxtaposition to and in communicationwith at least one of the sulfur layers. The multilayered cathode of thisinvention preferably includes wherein the carbon containing layercomprises a carbon nanotube —COO⁻ moiety. The multilayered cathode ofthis invention comprising alternatively arranged layers of the sulfurcontaining layers wherein the sulfur containing layers comprise one ormore sulfur containing compounds and one or more sulfur-carbon-polymercomposites. Preferably, the multilayered cathode of this inventionincludes wherein the layers have porous cross-linked structures.

Another embodiment of this invention provides the multilayered cathode,as described herein, wherein at least one of the sulfur containinglayers is a sulfur-carbon nanotube polystyrene sulfonate polymer.

Another embodiment of this invention provides the multilayered cathode,as described herein, wherein the outermost layer of the positivelycharged polymer is a sulfur polyaniline polymer.

In yet another embodiment of this invention, a lithium sulfur battery isdisclosed having at least one multilayered cathode and at least oneanode, wherein the multilayered cathode comprises at least one currentcollector working electrode having a surface comprising a carboncontaining layer; two or more sulfur containing layers wherein at leastone of the sulfur layers is located in juxtaposition to and incommunication with the carbon containing layer; and at least oneoutermost layer comprising a positively charged polymer for forminginterconnected layers of the sulfur containing layer, the carboncontaining layer, and the polymer, wherein the outermost layer is injuxtaposition to and in communication with at least one of the sulfurlayers. Preferably, the lithium sulfur battery, as described herein,includes wherein the carbon containing layer of the multilayered cathodecomprises a carbon nanotube —COO⁻ moiety (i.e. an example of afunctionalized carbon nanotube). Most preferably, the lithium sulfurbattery of this invention, as described herein, includes wherein themultilayered cathode comprises alternatively arranged layers of thesulfur containing layers wherein the sulfur containing layers compriseone or more sulfur containing compounds and one or moresulfur-carbon-polymer composites.

Another embodiment of this invention provides a lithium sulfur battery,as described herein, including wherein the layers of the multilayeredcathode have porous cross-linked structures.

Another embodiment of this invention provides a lithium sulfur battery,as described herein, includes wherein at least one of the sulfurcontaining layers of the multilayered cathode is a sulfur-carbonnanotube polystyrene sulfonate polymer.

Another embodiment of this invention provides a lithium sulfur battery,as described herein, including wherein the outermost layer of thepositively charged polymer of the multilayered cathode is a sulfurpolyaniline polymer.

In yet another embodiment of this invention, a method of making amultilayered cathode for a lithium sulfur battery is provided. Thismethod of making a multilayered cathode for a lithium sulfur batterycomprises employing at least one of the methods selected from the groupconsisting of (1) a layer-by-layer (LbL) method, (2) a step-by-stepelectrophoretic deposition (EPD) method, (3) a spin-assisted assemblytechnique, and (4) an alternately misting method, to produce amultilayered sulfur composite cathode.

Preferably, the method for making a multilayered cathode for a lithiumbattery comprises (a) providing a sulfur carbon nanotubepolystrenesulfonate composition dispersed in water for forming a sulfurcarbon nanotube polystyrene dispersion; (b) providing a sulfurizedpolyaniline composition dispersed in water for forming a sulfurizedpolyaniline dispersion; (c) providing a current collector having asurface comprising a carbon coating; (d) immersing the current collectorhaving the carbon coating into the sulfurized polyaniline dispersion toform a sulfurized polyaniline coated current collector; and (e)immersing the sulfurized polyaniline coated current collector into thesulfur carbon nanotube polystyrene dispersion for forming one layer ofthe sulfurized polyaniline and the sulfur carbon nanotubepolystyrenesulfonate treated current collector; and (f) repeating thesteps (d) and (e) one or more times to form one or more additionallayers of the sulfurized polyaniline and the sulfur carbon nanotubepolystyrenesulfonate upon the treated current collector. This methodincludes wherein the carbon coating comprises one or more of afunctionalized porous carbon, graphite, grapheme, carbon nanoparticles,carbon nanotubes, carbon fibers, and carbon rods. The functionalizedporous carbon is a carbon nanotube functionalized with a COO⁻ group toform a carbon nanotube COO⁻. This method preferably includes wherein thecurrent collector is one or more selected from the group consisting ofan aluminum substrate, a copper substrate, a nickel substrate, and aconductive glass.

While described in more detail below, one embodiment of this inventionemploys a LbL technique to fabricate the multilayered sulfur cathodes.First, we prepared two kinds of uniform dispersions with oppositecharges. The first one was PANI/SPANI (SPANI) that with positive chargesin the de-ionic water dispersion. The second one was functionalizedcarbon nanotube and sulfur composites that coating with a thin layer ofPSS (S-CNT/PSS) in the de-ionic water dispersion. Second, themultilayered cathodes were fabricated on aluminum current collectors byalternate adsorption of negatively charged S-CNT/PSS and positivelycharged SPANI. Third, the multilayered cathodes were heated at vacuumoven at 95° C. (Centigrade) for 5 h (hours). The fabrication process isshown in FIG. 1. FIG. 1 sets forth a schematic illustration for thepreparation of multilayered sulfur composite cathodes by LbL. Thealuminum substrate was treated with negative charges before LbL process.

One use for the process of this invention is in the film fabrication forlithium sulfur batteries. Another use for the process of this inventionis for carbon silicon composite anode fabrication.

This invention solves the problems associated with current knowntechnology. The present invention mitigates the capacity fading and lowpower density problems that exist in sulfur cathodes as part of lithiumsulfur batteries. The multilayered cathodes of this invention may beused for lithium sulfur batteries to provide great power and energydensities, stable cycling performances and low costs.

The multilayered cathodes of this invention produce synergistic effectsfrom the intimate contact between the selected components and lead toimproved sulfur cathodes for LSBs. First, the multilayered cathodes ofthis invention have increased power density of LSBs. Since sulfurcathodes involve multi-step reaction, Li— ion and electron transport isan important factor. Li-ion transport within a binder-free multilayeredfilm may be selectively tuned through the creation of a nanoporousnetwork. The empty pores act as reservoirs for liquid electrolytescapable of fast Li-ion conduction. Meanwhile, the highly cross-linkedSPAN1 and CNT facilitate electrical conductivity and, to a lesserextent, Li-ion transport. Second, the multilayered cathodes of thisinvention have improved energy density of LSBs. The control of cathodestructure ensures a homogeneously sulfur distribution in discretelayers, which provide huge reactive interfacial areas that allowconvenient incorporation and manipulation of sulfur into the selectedlayers. While the multilayers-electrolyte interfaces may be tuned,enhancing electronic and ionic conduction across the interfaces and thusleading to a maximization efficiency of sulfur. Third, the multilayeredcathodes of this invention effectively block polysulfide anions whilethe active material's functions and properties remain unaltered andionic/electronic transfer limitations are eliminated. The positivecharges on SPANI interact with polysulfide anions and reduce theirdissociation from the multilayered structure. The PSS polymers formdense protective films to trap polysulfides. On the other hand, theSPANI and functionalized CNTs incorporated within the multilayeredstructure serve as chemical reaction sites for sulfur and intermediatepolysulfides to ensure a more complete redox process. The structureallows for reversible in situ deposition of intermediate polysulfidespecies during discharge and their corresponding transformation duringcharge within the homogeneous functional group matrices, which furtherattract polysulfide anions from “leaking” of the multilayered cathodes.Fourth, the multilayered cathodes enhance stability of LSBs. The porousstructure yields the mechanical properties, which accommodate the volumechange and the corresponding strains accumulated in the cathodes. Inaddition, the SPANI shows great electrochemical performances thatstabilize the multilayered cathodes.

In one embodiment of this invention, the multilayered sulfur compositecathode of this invention,

(1) sulfur is combined with carbon or polymer to form sulfurized carbon,sulfurized polymer, carbon sulfur composite and polymer sulfurcomposite.

-   -   (a) The carbon material species include functionalized-porous        carbon, graphite, graphene, carbon nanoparticles, carbon        nanotubes, carbon fibers, or carbon rods.    -   (b) The polymers include polyaniline (PANI), polystyrene        sulfonate (PSS), poly (ethylene oxide) (PEO), polyethylene        glycol) (PEG).

(2) The current collector includes aluminum grid, aluminum foil,aluminum foam, copper foil, nickel foil and conductive glass.

(3) The alternative layers could include sulfur composite/compound orother polymers without sulfur.

(4) The multilayered sulfur composite cathodes may be fabricated by (a)LbL technique; (b) step-by-step electrophoretic deposition (EPD) method;(c) spin-assisted assembly technique, or a (d) an alternately mistingmethod (4a-d are procedures well known by those persons skilled in theart).

(5) The post-heated temperature for the multilayered sulfur cathodes isnot limited to 95° C. and the heated time is not limited to 10 h.

It is known by those skilled in the art that LSB suffers from incompletereduction of elemental sulfur to lithium sulfide, severe capacityfading, and low power density during multiple cycles, mainly originatingfrom the inherent challenges of its chemistry.^([)*^(1]) One of thechallenges is related to the multi-step electrochemical reactions fromS₈ to Li₂S where the intermediate polysulfides can easily dissolve intoliquid electrolytes, and this results in the so-called polysulfideshuttle effects: the dissolved polysulfides diffuse to the Li anodewhere they get reduced and then diffuse back to the sulfurcathode.^([)*^(2]) During these parasitic shuttle processes, the activematerial is irreversibly consumed and nonconductive sulfur crystals areaccumulated, leading to decreased capacity retention. Meanwhile, thechanges in cathode morphology induces strain inside the electrode andthe passivation of Li anode leads to an increase in impedance barrier,both of which also reduce the cyclic lives of batteries.^([)*^(3,)*⁴] Asecond challenge for LSBs is that the ionic and electric insulatingsulfur and low-order polysulfides (LPS, i.e. Li₂S_(n), n≦3) can resultin low energy and power density. The complete conversion of S₈ to Li₂Sis difficult since even a thin layer of LSP covering on the surface ofcathode can greatly inhibit lithiation and lead to rapid voltagedecrease.^([)*^(5,)*^(6,)*^(7]) Another challenge for LSBs is their highvolume change during cycling, which results in electrical isolation ofactive materials and therefore, fast capacity decay during multiplecycles.^([)*^(8])

The multilayered cathodes of the present invention produced synergisticeffects from the intimate contact between the selected components andled to improved sulfur cathodes for LSBs. First, the multilayeredcathodes of the present invention increased power density of LSBs. Sincesulfur cathodes involve multi-step reaction, Li-ion and electrontransport is an important factor. Li-ion transport within a binder-freemultilayered film could be selectively tuned through the creation of ananoporous network. The empty pores acted as reservoirs for liquidelectrolytes capable of fast Li-ion conduction. Meanwhile, the highlycross-linked SPAN1 and CNT facilitated electrical conductivity and, to alesser extent, Li-ion transport. Second, the multilayered cathodes ofthe present invention improved energy density of LSBs. The control ofcathode structure ensured a homogeneously sulfur distribution indiscrete layers, which provided huge reactive interfacial areas thatallow convenient incorporation and manipulation of sulfur into theselected layers. While the multilayers-electrolyte interfaces can betuned, enhancing electronic and ionic conduction across the interfacesand thus leading to a maximization efficiency of sulfur. Third, themultilayered cathodes of the present invention effectively blockpolysulfide anions while the active material's functions and propertiesremain unaltered and ionic/electronic transfer limitations wereeliminated. The positive charges on SPANI interact with polysulfideanions and reduce their dissociation from the multilayered structure.The PSS polymers form dense protective films to trap polysulfides. Onthe other hand, the SPANI and functionalized CNTs incorporated withinthe multilayered structure serve as chemical reaction sites for sulfurand intermediate polysulfides to ensure a more complete redox process.The structure allows for reversible in situ deposition of intermediatepolysulfide species during discharge and their correspondingtransformation during charge within the homogeneous functional groupmatrices, which further attract polysulfide anions from “leaking” of themultilayered cathodes. Fourth, the multilayered cathodes of the presentinvention enhanced stability of LSBs. The porous structure yields themechanical properties, which could accommodate the volume change and thecorresponding strains accumulated in the cathodes. In addition, theSPANI showed great electrochemical performances that stabilized themultilayered cathode.

EXAMPLES Example 1 Preparation of Multilayered Sulfur Composite Cathodes

First, SPANI (sulfur polyaniline) was treated with NH2OH solution at 70°C. for 2 h, and S-CNT (sulfur-carbon nanotube) was mixed withpoly(styrenesulfonate) (PSS, molecular weight (MW)˜70,000,Sigma-Aldrich) solution for 2 h (hours). These treated powders were thensonicated for 6 h in deionized water separately to form uniformdispersions. The pH values of both solutions were adjusted to 3.5 andthe solutions were sonicated for 3 h before LbL assembly. The purpose ofintroducing polystyrene sulfonate (PSS) here was to facilitate thegrowth of the multilayer films via electrostatic interactions betweenS-CNT and SPANI. Details of LbL assembly of electrodes are well known bythose persons skilled in the art. Assembled multilayered cathodes ofthis invention were dried in air and then treated at 90° C. (Centigrade)in a vacuum oven for 5 h to be prepared for cell assembling.

Surface Morphology

FIG. 3 shows scanning electron microscope (SEM) images of themultilayered sulfur composite cathode of this invention. Thecross-section SEM showed that S-CNT/PSS and SPANI were well distributedthroughout the multilayered film (FIG. 3a ), and the outermost surfacewas covered with SPANI (FIG. 3b ). In addition, the cathode has a smoothfibrous morphology and a 3D interconnected network with a porousstructure (FIG. 3b ). Such porous structure allows high electrodematerial loading with better electrode-current collector adhesion.

Cycle Performance

For the charge-discarge analysis, two-electrode coin cells (2032) withLi foil as counter electrode were assembled in an argon-filled glove box(Labstar). The electrolyte consisted of 1.0 M LiTFSI and 0.15 M AgNO₃that dissolved in dioxolane (DOL) and dimethyl ether (DME) (1:1, v/v),and a micro-porous separator (Celgard) was used between the multilayeredcathode and Li foil. Then the assembled cells were discharged andcharged at three current densities of 550, 1300, and 1950 mAg⁻¹ between1 and 3 V (vs. Li/Li+) using an Arbin battery test station (BT2000).

FIG. 4 shows the charge-discharge profiles of the multilayered cathodesincluding cycling performance, discharge capacity, coulombic efficiencyand voltage information. The multilayered sulfur cathodes retainedreversible gravimetric capacities of 920, 780, and 600 mAhg⁻¹ at acurrent density of 550, 1300, and 1950 mAg⁻¹, respectively (FIG. 4a ).The capacities were calculated based on the whole materials' weight oncathodes. The multilayered cathodes exhibited high reversible capacitiesand long cyclic lifetimes. For instance, at the current of 550 mAg⁻¹,the multilayered cathodes had a high reversible capacity of 920 mAhg⁻¹and a high retention capacity of 605 mAhg⁻¹ was still obtained evenafter 420 cycles (FIG. 4b ). In addition, the multilayered cathodes hada high coulomb efficiency of about 100%. The voltage profiles ofdischarge/charge cycles of multilayered cathodes showed typicalcharacteristics of combination of SPAM and S-CNT (FIGS. 4c and 4d ).

Example 2

Lithium-Sulfur (Li—S) batteries suffer from major problems including lowactive material utilization, poor cycling performance, and lowefficiency, mainly due to the high solubility of intermediatepolysulfides and their side-reactions with electrolyte solvents and theLi-anode. Here, we report the development of advanced, multilayered,sulfur electrodes comprising alternately arranged, negatively chargedS-carbon nanotube layers and positively charged S-polyaniline layersthat effectively immobilize polysulfides in the multilayered cathodespreventing polysulfide migration onto the Li-anode. The use of alayer-by-layer self-assembly technique leads to a 3-D porous cathode viaelectrostatic attraction, and enables the fabrication of remarkablyimproved Li—S cells with a reversible capacity of 1100, 900, and 700mAhg⁻¹ at 0.3, 0.6, and 1 C current, respectively, while also deliveringan average Coulombic efficiency of 97.5% and providing a lifetime inexcess of 600 cycles. The results provide important progress towards theunderstanding of the role of multilayered cathodes with positive chargestoward the realization of high efficiency and long cycle performance forLi—S batteries.

Sulfur's high theoretical capacity of 1672 mAhg⁻¹, a tenfold greatercapacity versus today's lithium ion batteries, make lithium-sulfurbatteries an attractive candidate for meeting increasing demand forhigher energy density, lower cost, and environmentally friendly energystorage devices. However, Li—S chemistry is inherentlychallenging.^([1, 2]) The formation of soluble, long-chain polysulfides(Li₂S_(n), n>4) during discharge/charge cycling common to mostpresent-day Li—S battery designs leads to the irreversible loss ofactive materials from the cathode into the electrolyte and onto theLi-anode. The reduced polysulfides at the anode causes a continuousevolution of porous Li metal structure, and thus leads to unstablesolid-state electrolyte interface layers, damaging long-term cellperformance and presenting safety issues. Meanwhile, changes in thecathode morphology resulting from the 80 percent change in materialvolume during discharge/charge cycling induces strain inside thecathode, leading to low efficiency and fast capacity decay of cycling.Further, the detachment of Li_(x)S from the carbon surface duringcycling because of the high volume change of sulfur results in lowsulfur utilization and severe capacity degradation. The chemistryresults in uncontrollable deposition of lithium sulfide species on boththe cathode and anode surfaces, significantly inhibiting furtherlithiation, leading to low sulfur utilization.^([3-7])

To address these problems, various sulfur-carbon/polymer composites havebeen used to trap soluble polysulfides and provide fast kineticreactions.^([8-13]) Other approaches focus on Li anode^([14, 15]) andelectrolyte designs^([16]), aiming to prevent the undesirableinteractions between polysulfides and the highly reductive Li-anode.However, these improvements have their own limitations. For example, thedetachment of highly polar polysulfides from non-polar carbon conductiveagents during discharge/charge and their subsequent dissolution into theelectrolyte is believed to be an important factor in capacitydegradation.^([9-10, 17-18]) These approaches require the significantuse of binders, conductive agents, and modifying precursors in thecathode and thus neutralize the advantages of Li—S batteries.

In this invention, we provide a process of making a multilayered sulfurcomposite cathode. Preferably, this process employs the layer-by-layer(LbL)-process^([19]) fabrication of efficient, multilayered sulfurcathodes to address the challenges of Li—S batteries. The multilayeredcathodes were fabricated on aluminum current collectors by alternateadsorption of negatively charged S-carbon nanotubes polystyrenesulfonate (S-CNT-PSS⁻) and positively charged S-polyaniline (SPANI)-NH⁺as shown in FIG. 1a . Polyaniline (PANI) was deposited as the outermostlayer to prevent direct contact between sulfur and the electrolyte. BothCNT and PANI are attractive choices as sulfur carriers because of theirhigh electronic and ionic conductivities, strong affinity, and highloading of sulfur and polysulfides.^([20-27]) interconnected layers ofS-CNT and SPANI with their multiple pores served as high efficiencybinders, conductive agents, and 3-D mechanical scaffolds for theefficient use of sulfur. S-CNT layers sandwiched between two positivelycharged SPANI layers favored the immobilization and attachment ofpolysulfide anions within the highly conductive structures by providingstrong interactions. The sandwich-like porous structures acted asself-control poly-shuttle frameworks by forming physical and chemicalC—S bond barriers that retarded polysulfide migration from the cathodetoward the Li-anode as shown in FIG. 5b . These advanced multilayeredcathodes contained 67.5 wt. % of sulfur, enabling very high and stablereversible specific capacities of 1100, 900, and 700 mAhg⁻¹ at a currentdensity of 0.3, 0.6, and 1 C, respectively, and provided adischarge/charge lifetime in excess of 600 cycles with an averageCoulombic efficiency of 97.5%.

FIG. 5a shows a schematic diagram of multilayered cathode fabricated bythe LbL-process and functions of each component. FIG. 5b shows aschematic diagram of the self-control poly-shuttle process in themultilayered cathode.

Results Characterizations of the Multilayered Cathodes and RelatedMaterials

Materials. The S-CNT and SPANI were synthesized using functionalized CNT(FCNT) and PANI. The pristine FCNT tended to agglomerate due to strongvan der Waals interactions) (FIG. 6a ). However, these interactionsappeared to weaken after a thin layer of sulfur (˜20 nm thick) wasuniformly coated on the surface of the FCNT (FIG. 6b ). The depositionof sulfur onto the FCNT was evaluated by X-ray diffraction (XRD). Noobvious characteristic peaks of FCNT were observed for S-CNT (FIG. 10a). A monoclinic sulfur phase was detected by XRD in S-CNT after heatingat 159° C. (Centigrade) for 8 h (hour) then 300° C. for 1.5 h. At 300°C., S₈ rings may break down into much more active, smaller sulfurallotropes (S₆ and S₂) which readily dehydrogenate and react with carbonor substitute for oxygen atoms to form covalently bondedsulfur.^([23, 28]) The C—S bonds were verified by the two additionalpeaks at 740 cm⁻¹ and 933 cm⁻¹ in the Fourier transform infraredspectroscopy (FTIR) analysis (FIG. 10b ), since it is known that S₈shows no vibrational activity in the 900 to 2000 cm⁻¹ range. The sulfurcontent in the S-CNT was found to be 76.9 wt. % by thermo gravimetricanalysis (TGA) (FIG. 10c ). TGA results indicated that the weight-lossand weight-loss temperature of S-CNT was higher than those of S/CNTcomposites, suggesting a promoted affinity and interaction betweensulfur and FCNT. The typical morphology of the SPANI was shown in FIG.6c where bulk sulfur particles were also found on SPANI surfacesattributed to strong capillary force during the post-heat treatment.X-ray photoelectron spectroscopy (XPS) was conducted on the SPANIpolymer and the fitted curves indicated the back-chains of PANI werechemically modified and physically coated with sulfur (FIGS. 11a and 11b). TGA results indicated the sulfur content in SPANI was 65.4% (FIG. 11c).

The ultrahigh aspect ratio and good mechanical strength of FCNTs andPANIs create a multilayered cathode of this invention having a robuststructure that possesses abundant interconnected channels through whichLi-ions may pass (FIGS. 6d-6f ). These channels form 3-D porousframeworks that favor the penetration of electrolytes. Since sulfur isinvolved in multi-step reactions during discharge and charge, Li-ion andelectron transport is an important factor. The empty pores in themultilayered cathode acted as reservoirs for liquid electrolytes capableof fast Li-ion conduction. Meanwhile, the highly intertwined PANIs andFCNTs facilitated electrical conductivity and, to a lesser extent,Li-ion transport. The multilayered structure forms a 3-D integratedskeleton and the discrete layers ensure a homogeneous sulfurdistribution. The skeleton provides huge reactive interfacial areas thatallow convenient incorporation and manipulation of sulfur. The structurefacilitates electronic and ionic conduction across the multilayeredinterfaces between the discrete layers and the electrolyte, maximizingthe efficiency of the sulfur in combining with lithium. The thickness ofthe multilayered cathode increased approximately linearly with theincreasing number of bilayers; the cathode with 90 bilayers had athickness of 35.3 μm with a material density of 2.75 mg·cm⁻² after heattreatment (FIG. 6g ). XPS analysis of the multilayered cathodes afterheat treatment revealed significant amounts of S₈ and C—S bonds withinthe cathodes (FIG. 6h ). The peak at 164.4 eV in the S 2p3/2 spectrumindicates elemental sulfur, while the peak at 165.4 eV in the S 2p1/2spectrum suggests that S atoms were linked to a benzenoid ring (SPANI)and a quinoid ring (sulfurized CNT). The small peak at 168.5 correspondsto PSS. The atomic composition of the multilayered cathode was found tobe 29.2 wt. % carbon, 64.1 wt. % sulfur, 4.6 wt. % oxygen and 2.1 wt. %nitrogen. TGA results indicated the sulfur content in the whole cathodewas 67.5% (FIG. 6i ). FIG. 6 shows the characterizations of multilayeredcathodes and related materials. (6 a-f) are scanning electron microscopy(SEM) images of (6 a) FCNT, (6 b) S-CNT, (6 c) SPANI, (6 d) theoutermost layer, PANI, (6 e) SPANI layer, and (6 f) S-CNT layer. FIG. 6(g) shows the area mass density (mg cm⁻²) and thickness (μm) vs.bilayers of multilayered cathodes on aluminum current collectors. FIGS.6(h) and 6(i) show XPS and TGA analysis for the multilayered cathodes.In FIG. 6(i), the sample of FCNT & PANI was used as a backgroundcomparison for the calculation of sulfur content in the multilayeredcathodes. The sample was prepared with the same method for fabricatingthe multilayered F-CNT-SPANI cathodes.

Electrochemical Performance of the Multilayered Cathodes of thisInvention

The rate capability of the multilayered cathodes is shown in FIG. 7a .The C rates specified in this study are based on the theoreticalcapacity of sulfur, with 1 C=1675 mAg⁻¹. The initial discharge capacityreaches 1346 mAhg⁻¹ at 0.1 C, which is 80.4% of the theoretical valuefor sulfur. A reversible capacity of 1014 mAhg⁻¹ was observed at the300^(th) cycle, corresponding to 75.3% capacity retention. The resultsdemonstrated the superiority of the multilayered structure in enhancingthe active material utilization. As current density varied from 0.1 to2.5 C, the multilayered cathodes still displayed reasonable capacityalthough capacity decreased gradually due to polarization effect. Evenat a rate of 2.5 C, the cell capacity exceeded 600 mAhg⁻¹, demonstratingexceptional rate performance and robustness of the structures. Thelong-term cycling behavior and Coulombic efficiency of Li—S cellscontaining multilayered cathodes at different current densities is shownin FIGS. 7b and 7c . Significantly improved cycling stability wasobserved at all rates studied. For instance, at 0.3 C, the multilayeredcathodes had a high reversible capacity of 1100 mAhg⁻¹. A reversiblecapacity of 818 mAhg⁻¹ was obtained even after 600 cycles, correspondingto 74.4% of capacity retention with an average Coulombic efficiency of97.5%.

At 0.3, 0.6, 1, and 2.5 C, the decrease in the first few cycles followedby an increase in discharge capacity were observed, indicating themultilayered cathodes required an activation step. The decrease wasprobably caused by the catalytic reduction of electrolyte solvents onthe fresh surfaces of the multilayers, and the formation of solidelectrolyte interface films on Li-anodes. The increase was related tothe solubility of polysulfides. Initially, the cathodes contained bulksulfur, which could not completely react at the end of discharge. Aftera few cycles, the electrolyte infiltrated into the internal layers andthe bulk sulfur reacted and pulverized, leading to small sulfurparticles. Subsequently, the cells reached steady state and showedstable cyclic properties. However, there was no activation process at0.1 C, indicating a threshold current for the multilayered cathodes inthe first a few cycles, below which most of sulfur is reacted and abovewhich a significant amount of sulfur remains at the end of discharge.

The high capacity and excellent cycling stability of multilayeredcathodes may be explained by their unique layered porous structures.Unlike sulfur cathodes fabricated by slurry-coating with their inferiorefficiency and low capacities (FIG. 12) due to polysulfide shuttling andpoor contact between sulfur and carbon during discharge/charge, themultilayered porous frameworks led to improved Li—S cell performance forthree reasons: (i) the multilayered cathodes provided adequatehigh-efficient electron and Li-ion conduction channels formed by theabundance of 3-D pores (FIGS. 6c and 6d ); (ii) the multilayeredcathodes provided strong affinity of polysulfides and reduced theirdissociation from the selected layers during cycling (FIGS. 7d and 7f ).The intimate contact layers further attracted polysulfide anions andprevented the anions from “leaking” out of the multilayered structure(FIG. 7e ); and (iii) The porous frameworks yielded mechanicalproperties that accommodated the volume change of the sulfur and thecorresponding strains accumulated in the cathodes, thus leading toimproved cycling stabilities.

FIG. 7a shows rate performance of multilayered cathodes, and long-termcycling performance of multilayered cathodes at (FIG. 7b ) 0.3, 0.6 and1 C, and (FIG. 7c ) 0.1 and 2.5 C. SEM images reveal that the dischargeproducts are kept on the cathode structure at 10^(th) cycle (FIG. 7d topview and (FIG. 7e cross-section) and (FIG. 7f ) 50^(th) cycle.

Electrochemical Reaction Processes in the Multilayered Cathodes of thisInvention

The electrochemical reaction mechanism of sulfur in multilayeredcathodes was revealed using the cyclic voltammetry (CV) at a scan rateof 0.05 mVs⁻¹. As shown in FIG. 8a , the fresh multilayered cathodefeatured three reduction current peaks at around 2.35, 2.1, and 1.8 V.The first two narrow peaks showed typical characteristics of two-stepreduction of sulfur from solid-liquid (S₈—S₆ ²⁻) and liquid-solid (S₆²⁻—Li₂S₂) phase transitions. The third broad reduction current peak at1.8 V was lower than the potential of S₄ ²⁻ to Li₂S₂ reaction at around2.1 V. For comparison, we plotted the CV curves of SPANI and S-CNT-basedcells (FIG. 13). Both S-CNT and SPANI showed typical sulfurcharacteristics with two reduction peaks at 2.44/2.04 and 2.3/1.97 V,respectively. These observations indicate that the multilayered cathodesexperienced a new reaction represented by the reduction peak at 1.8 V,which is possibly attributed to the reaction from Li₂S₂ to Li₂S. Asshown in FIG. 14, the four characteristic peaks at 23°, 31°, 45°, and50° in the XRD pattern indicate the existence of Li₂S. However, the CVcurves at the 50^(th) cycle showed significantly different behavior. Thefirst two reduction peaks showed in the initial cycle were substitutedby a new broad peak centered at 2.2 V in the 50^(th) cycle. Most likelythe high potential polarization between soluble, high-order polysulfides(HPS, i.e. Li₂S_(n), n≧3) and insoluble, low-order polysulfides(Li₂S_(n), n≦2) caused an overlap of the two possible reduction peaks.The continuous CV scan of the multilayered cathodes shown in FIG. 15demonstrates the gradual changes during the electrochemical reactionprocesses.

The discharge/charge profiles in FIG. 8b exhibit three dischargeplateaus at 2.3, 2.1, and 1.9 V and two discrete charge plateaus at 2.3and 2.4 V, which were consistent with the CV analysis. The upperdischarge plateau at 2.3 V corresponded to the reduction of sulfur intosoluble lithium polysulfides, indicating the effective utilization ofsulfur. The capacities corresponding to this plateau at 1^(st), 50^(th),and 150^(th) cycles were identical, demonstrating the effectiveness ofthe multilayered cathode in trapping soluble polysulfides and enhancingthe utilization of sulfur. The multilayered cathodes reduced sulfur toLi₂S₂ and Li₂S before possible soluble polysulfides could diffuse out ofthe multilayered cathode into the liquid electrolytes. When sulfur ineach layer is reduced upon full discharge, the strong affinity ofpolysulfides for the sandwich-like porous frameworks is vital forretaining the active mass and electrical contact of sulfur/polysulfideswith the conductive framework. SEM images of the multilayered cathodesin the discharged state revealed that the discharge products, namely(name the discharge products here), were uniformly kept within thecathode structure to form thick layers instead of discrete particles,implying the strong interaction between polysulfides and themultilayered structure. As shown in FIGS. 8c-f , after the activationprocess, the initial bulk sulfur dissolved into small sulfur particlesor formed thin sulfur layers distributed in the discrete layers (FIGS.8c and 8d ). The multilayered cathode had adequate pores to store sulfurand polysulfides (FIGS. 8e and 8f ), and until the 500^(th) cycle, theporous framework was covered with thick sulfur/polysulfide layers. Aspreviously discussed, the unique multilayered structure in the cathodesprevented the dissolution of polysulfides as follows: (i) the tightlyintertwined surface groups not only fixed polysulfides and resulted inenhanced maintenance of the structural integrity of the cathode duringcell operation, but also provided special reaction sites for sulfur(i.e. C—S bonds) to facilitate a more complete transformation from S₈ toLi₂S₂ and Li₂S; (ii) the electrostatic interaction between thealkylammonium cations and polysulfide anions efficiently entrappedpolysulfides during the repeated cycles, leading to excellent cyclingstability; and (iii) the highly porous 3-D framework achieved highutilization of sulfur and fast kinetics by providing highly efficientelectron and ion pathways.

FIG. 8a shows CV scans of multilayered cathodes (scan rate was 0.05mVs⁻¹); FIG. 8b shows voltage profiles of multilayered cathodes at1^(st), 50^(th) and 150^(th) cycles; and top surface characterization ofmultilayered cathodes arte shown in FIG. 8c at 150^(th) cycle, FIG. 8dat 250^(th) cycle, FIG. 8e at 350^(th) cycle, and FIG. 8f at 500^(th)cycle.

Electrochemical impedance spectra (EIS) analysis was performed tofurther evaluate the mulilayered cathodes. As shown in FIG. 9a , the EISspectra comprised of two semicircles at the high frequency region and aninclined tail in the low frequency region. The real axis intercepting atthe high frequency represented the electrolyte resistance. Thesemicircle from high to medium frequency corresponded to the SEI layerresistance, and the second semicircle at medium frequency was related tothe charge transfer resistance. The short inclined line in low frequencyregions was due to ionic diffusion within the cathode. The interfacialcharge-transfer resistance was recognizable from the second semicircledue to the redox formation of soluble polysulfides and insolubleshort-chain polysulfides. The resistances at the 50^(th) and 150^(th)cycles were identical, indicating a stable SEI layer on the lithiumsurface and a smooth charge transfer of lithium ions in the cell. Thesefindings might indicate that there was little or no polysulfideshuttling in the first 150 cycles since otherwise, the dissolvedpolysulfides would migrate toward the Li-anode at which point they wouldbe reduced to LPS and irreversibly precipitate onto the Li-anode surfacewhich would inhibit charge transfer of lithium ions thereby leading toan increase in cell impedance. Cathode kinetics and charge-transferpolarization accounted for the majority of the voltage loss in cells.The multilayered cathodes had low resistances, which presented an idealopportunity to create intimate organic-inorganic interfaces forefficient electrochemical reactions in Li—S batteries. At the 500^(th)cycle, by contrast, both the interfacial resistance and charge transferresistance increased. The reactions of sulfur resulted in greatmorphological changes at the 500^(th) cycle, leading to (i) obviouscracks and (ii) formation of insulating crystal slabs on the surfaces ofthe multilayered cathodes (FIG. 9b ). Both the nonconductive species andthe cracks caused high resistances within the cell. FIG. 9a shows theEIS analysis of the Li—S cells with multilayered cathodes, and FIG. 9bshows SEM image of cathode surface after 500 cycles.

This example provides that a multilayered sulfur composite cathode withhigh sulfur content of 67.5 wt. % was fabricated using the LbL-assemblymethod. The interconnected PANI and FCNT layers served as an electricalconductive network and the abundance of pores served as ionic conductivepathways. SEM images indicated sulfur was well distributed in discretelayers before cycling, which enhanced sulfur utilization by providingintimate contacts of sulfur to the highly conductive selected layers.The discharge/charge voltage profiles and the CV scans, combined withthe EIS, XPS and FTIR analyses, revealed that the multilayered cathodeswere highly efficient in fixing and trapping soluble polysulfides duringcell operation. As a result, the multilayered cathodes provided a longlifetime of more than 600 cycles with an average Coulombic efficiency of97.5% under a variety of discharge/charge current densities. Weattributed the high rate capability and cycling stability to the uniquemultilayered porous structures which provided adequate electron/ionconductive pathways and strong affinity of polysulfides for multilayeredframework. This unique composite favored a much more complete sulfurtransformation from S₈ to Li₂S or Li₂S₂ before possible solublepolysulfides could diffuse into electrolyte leading to excellent celloperation with high efficiency, good reversibility, and fast kinetics.SEM images showed no structural damage to the multilayered cathode untilthe 500^(th) cycle, indicating robust mechanical properties of themultilayered cathode fabricated by the LbL technique. The sulfurconfinement mechanisms and structural superiority of the multilayeredcathodes were sufficient to retard polysulfide dissolution therebyresulting in long-term cycling stabilities while achieving highcapacities with excellent rate capabilities. LbL-fabricated,multilayered cathodes offer great promise for the ubiquitous applicationof low-cost, long-lived, high energy density, high power Li—S batteriesfor electric vehicle systems and flexible and thin-film devices.

Methods (1) Materials. A. Preparation of S-CNT Composites.

Multi-walled carbon nanotubes (CNTs, 110-170 nm in diameter and 5-9 μmin length, Sigma-Aldrich, St. Louis, Mo.) were soaked in nitric acid (70wt. %) and sulfuric acid (98 wt. %) (v 1:3) in an ultrasonic containerfor 1 h, kept in an oven of 70° C. for 2 h, and then rinsed withdistilled water seven times to get FCNT. The FCNTs were dispersed intosodium dodecyl sulfate (SDS) aqueous solution (Sigma-Aldrich).Meanwhile, sulfur powder (99.98%, Sigma-Aldrich) was dissolved intetrahydrofuran (THF, Sigma-Aldrich) to form a saturated solution. Next,the sulfur-saturated THF and FCNTs in SDS were mixed for 12 h undermagnetic stirring, then centrifuged. The supernatant was decanted andthe remaining materials were washed using deionized water three times toremove SDS. Finally, the as-prepared S-CNTs were mixed with sulfur (1:1wt. %) and treated in a vacuum oven at 159° C. for 8 h then at 300° C.for 1.5 h.

B. Preparation of SPANI.

PANI, sulfur monochloride, and aluminum chloride (99.99%), purchasedfrom Sigma-Aldrich, were soaked in acetonitrile in a sealed flask for 10h, washed using ether five times, and dried in a vacuum oven at 80° C.for 24 h to obtain chloride PANI. Sulfur and sodium sulfide were thenmixed in N,N-dimethylformamide (DMF) in a vacuum oven for 6 h to obtaindisodium pentasulfide (Na₂S₅). Next, the chloride PANI was soaked in aNa₂S₅ solution for 24 h in a vacuum oven, washed with deionized waterten times, and dried in a vacuum oven at 80° C. for 24 h to achieveinitial sulfurized PANI. Finally, the initial sulfurized PANI was mixedwith sulfur (1:1 wt. %) in carbon disulfide solution (CS₂,Sigma-Aldrich) for 2 h under magnetic stirring and heated in a vacuumoven at 280° C. for 2 h to obtain SPANI. (2) Fabrication of multilayeredcathodes using LbL self-assembly technique.

A. Aluminum Substrate Treatment.

In this work, the aluminum current collector was selected as thesubstrate for the LbL process. First, a thin layer of CNT-COO⁻ wasdeposited on the substrate by the electrophoretic deposition (EPD)technique.

B. LbL Process.

First, SPANI was treated with NH₂OH solution at 70° C. for 2 h, andS-CNT was mixed with poly(styrenesulfonate) (PSS, MW˜70,000,Sigma-Aldrich) solution for 2 h. These treated powders were thensonicated for 6 h in deionized water separately to form uniformdispersions. The pH values of both solutions were adjusted to 3.5 andthe solutions were sonicated for 3 h before LbL assembly. The purpose ofintroducing PSS here was to facilitate the growth of the multilayerfilms via electrostatic interactions. Details of LbL assembly ofcathodes can be found elsewhere^([29-32]). In brief, the processinvolves immersing the treated substrate into the SPANI suspension for 3min and then washing the substrate in deionized water for 30 sec; next,placing the SPANI-coated substrate into S-CNT suspension for 3 min(minutes) and then washing in deionized water for 30 sec (seconds).These steps are repeated until the desired number of layers is achieved.Finally, the assembled multilayered cathodes are dried in air and thentreated at 100° C. in a vacuum oven for 5 h. The cathode is now ready tobe assembled into a cell.

(3) XPS, XRD, and FTIR Measurements.

Kratos Axis Ultra XPS (Kratos Analytical) with a monochromatized Al KaX-ray source, PANalytical XRD, and FTS 7000 FTIR were used to analyzethe surface chemistry of S-CNT, SPANI, and the multilayered cathodes.Curve fittings of the XPS spectra were performed following aShirley-type background subtraction. The figures for XRD and FTIR wereplotted with Origin using the notable peaks as a reference.

(4) Electrochemical Measurements of Multilayered Cathode-Based Cells.

CR2016-type coin cells were used as the testing cells.—Lithium foilswere used as the anodes, Cellgard 2400 microporous membranes asseparators, 1.0 molL⁻¹ bis(trifluoromethane sulfonyl) imide (LiTFSI) and0.15 molL⁻¹ LiNO₃ dissolved in dioxolane (DOL) and dimethoxyethane (DME)(1:1, v/v) as electrolytes, and S-CNT/SPANI multilayered composite ascathodes. The cells were assembled in an Argon-filled glove box.Electrochemical measurements were performed galvanostatically between1.0 and 3.0 V at current densities of 550, 1300, 1950, and 6400 mAg⁻¹.Capacity was calculated based on the weight of all materials on thecathodes. CV experiments were conducted using a NOVA potentiostat atscan rates of 5, 0.5, and 0.1 mVs⁻¹. EIS measurements were carried outusing a NOVA electrochemical workstation in a frequency range between100 kHz and 100 mHz at a potentiostatic signal amplitude of 5 mV. Allexperiments were conducted at room temperature.

FIG. 10a shows XRD analysis of S-CNT, SPANI and FCNT. The S-CNT hastypical sulfur peaks, indicating the uniform deposition of onto CNTsurfaces. By comparison, SPANI shows both sulfur and PANI peaks,indicating that sulfur layers do not completely coat the PANI surface.FIG. 10b shows FTIR spectra of S-CNT. All the C—O and C═O peaks of S-CNTare smaller than that of FCNT, indicating the decrease of functionalgroups after reacting with sulfur. The two additional distinct peaks inthe S-CNT between 700 and 1000 indicate C—S bonds. FIG. 10c shows TGAand DSC analysis of S-CNT showing 76.9 wt. % of sulfur in the S-CNTcomposite. Compared with S/CNT, which here was synthesized using thetraditional method of heating a mixture of sulfur and CNT at 159° C. for10 hours, the S-CNT has a higher decomposition temperature (390° C.)than S/CNT (320° C.), suggesting a promoted affinity and interactionbetween sulfur and FCNT.

FIG. 11a shows XPS spectra of SPANI. FIG. 11b shows 2p spectra of SPANI.The raw and fitted curves indicate sulfur mainly existed as S—S and C—Sforms in SPANI. FIG. 11c shows TGA and DSC analysis of SPANI showing65.4 wt. % of sulfur in SPANI composite.

FIG. 12 shows cycling performance at various current densities for theslurry-coated cathodes that contain 10 wt. % of PVDF, 45 wt. % S-CNT and45 wt. % SPANI. Both the S-CNT and SPANI are the materials used in themultilayered sulfur cathode. There are not any carbon black or othercarbon conduct agents. The thickness of the film is about 40 um, whichis similar with the thickness of the multilayered film.

FIG. 13 shows CV data of S-CNT and SPANI based cathodes. The scan ratewas 0.05 mVs⁻¹. The S-CNT based cathodes contained 10 wt. % of PVDF and90 wt. % of S-CNT; the SPANI based cathodes contained 10 wt. % of PVDFand 90 wt. % of SPANI. Both S-CNT and SPANI show typical sulfur CVpeaks. The real line indicate the first cycle, and the dotted lineindicate the second cycle of the CV tests.

FIG. 14 shows XRD data of the multilayered cathode after 50^(th)discharge. The cathode was obtained in the glove box and washed by DOL.After drying in the glove box, it was transferred into a sealedoxygen-insulting box for ex-situ XRD testing.

FIG. 15 shows continuous CV scans of the multilayered cathodes for thefirst 3 cycles.

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It will be appreciated by those persons skilled in the art that changescould be made to the embodiments described herein without departing fromthe broad inventive concept thereof. It is understood, therefore, thatthis invention is not limited to the particular embodiments disclosed,but is intended to cover modifications that are within the spirit andscope of the invention, as defined by the appended claims.

What is claimed is:
 1. A multilayered cathode for a lithium sulfurbattery comprising: at least one current collector working electrodehaving a surface comprising a carbon containing layer; two or moresulfur containing layers wherein at least one of said sulfur layers islocated in juxtaposition to and in communication with said carboncontaining layer; and at least one outermost layer comprising apositively charged polymer for forming interconnected layers of saidsulfur containing layer, said carbon containing layer, and said polymer,wherein said outermost layer is in juxtaposition to and in communicationwith at least one of said sulfur layers.
 2. The multilayered cathode ofclaim 2 wherein said carbon containing layer comprises a carbon nanotube—COO⁻ moiety.
 3. The multilayered cathode of claim 1 comprisingalternatively arranged layers of said sulfur containing layers whereinsaid sulfur containing layers comprise one or more sulfur containingcompounds and one or more sulfur-carbon-polymer composites.
 4. Themultilayered cathode of claim 1 wherein said layers have porouscross-linked structures.
 5. The multilayered cathode of claim 1 whereinat least one of said sulfur containing layers is a sulfur-carbonnanotube polystyrene sulfonate polymer.
 6. The multilayered cathode ofclaim 1 wherein said outermost layer of said positively charged polymeris a sulfur polyaniline polymer.
 7. A lithium sulfur battery having atleast one multilayered cathode and at least one anode, wherein saidmultilayered cathode comprises at least one current collector workingelectrode having a surface comprising a carbon containing layer; two ormore sulfur containing layers wherein at least one of said sulfur layersis located in juxtaposition to and in communication with said carboncontaining layer; and at least one outermost layer comprising apositively charged polymer for forming interconnected layers of saidsulfur containing layer, said carbon containing layer, and said polymer,wherein said outermost layer is in juxtaposition to and in communicationwith at least one of said sulfur layers.
 8. The lithium sulfur batteryof claim 7 wherein said carbon containing layer of said multilayeredcathode comprises a carbon nanotube —COO⁻ moiety.
 9. The lithium sulfurbattery of claim 7 wherein said multilayered cathode comprisesalternatively arranged layers of said sulfur containing layers whereinsaid sulfur containing layers comprise one or more sulfur containingcompounds and one or more sulfur-carbon-polymer composites.
 10. Thelithium sulfur battery of claim 7 wherein said layers of saidmultilayered cathode have porous cross-linked structures.
 11. Thelithium sulfur battery of claim 7 wherein at least one of said sulfurcontaining layers of said multilayered cathode is a sulfur-carbonnanotube polystyrene sulfonate polymer.
 12. The lithium sulfur batteryof claim 7 wherein said outermost layer of said positively chargedpolymer of said multilayered cathode is a sulfur polyaniline polymer.13. A method of making a multilayered cathode for a lithium sulfurbattery comprising: employing at least one of the methods selected fromthe group consisting of (1) a layer-by-layer (LbL) method, (2) astep-by-step electrophoretic deposition (EPD) method, (3) aspin-assisted assembly technique, and (4) an alternately misting method,to produce a multilayered sulfur composite cathode.
 14. A method ofmaking a multilayered cathode for a lithium sulfur battery comprising:(a) providing a sulfur carbon nanotube polystrenesulfonate compositiondispersed in water for forming a sulfur carbon nanotube polystyrenedispersion; (b) providing a sulfurized polyaniline composition dispersedin water for forming a sulfurized polyaniline dispersion; (c) providinga current collector having a surface comprising a carbon coating; (d)immersing said current collector having said carbon coating into saidsulfurized polyaniline dispersion to form a sulfurized polyanilinecoated current collector; and (e) immersing said sulfurized polyanilinecoated current collector into said sulfur carbon nanotube polystyrenedispersion for forming one layer of said sulfurized polyaniline and saidsulfur carbon nanotube polystyrenesulfonate treated current collector;and (f) repeating said steps (d) and (e) one or more times to form oneor more additional layers of said sulfurized polyaniline and said sulfurcarbon nanotube polystyrenesulfonate upon said treated currentcollector.
 15. The method of claim 14 wherein said carbon coatingcomprises one or more of a functionalized porous carbon, graphite,grapheme, carbon nanoparticles, carbon nanotubes, carbon fibers, andcarbon rods.
 16. The method of claim 14 wherein said functionalizedporous carbon is a carbon nanotube functionalized with a COO⁻ group toform a carbon nanotube COO⁻.
 17. The method of claim 14 wherein saidcurrent collector is one or more selected from the group consisting ofan aluminum substrate, a copper substrate, a nickel substrate, and aconductive glass.